There are issues in the upstream source: 1. it ships binary files like images, rendered EPUB, PDF, and HTML. Those could be patches out of the orig.tgz relatively easily.
2. it ships vendored code in vendor/. this is trickier: the vendor directory could be patched out, but at least one dep (i2plib) is not packaged in Debian, and the others are hotwired elsewhere in the code (in `import .vendor...`), so would require patching in a couple places I filed a discussion upstream to see how open they would be to tweak this. I've pushed my work in https://salsa.debian.org/debian/rns so far A.